Fix #14371: FP redundantAssignment for nested union members

[DanTheMan2000]

No description provided.

[DanTheMan2000]

There were actually two variants of this false positive. The one from the ticket involved reading another union member inside a larger expression. The other case is where an assignment to a different union member sits between the two writes to the member being reassigned. Added tests covering both in testother.cpp.

[DanTheMan2000]

Added a couple of control tests to go with the false-positive ones, wanted to make sure the fix is scoped to the actual variable being reassigned.

[chrchr-github]

Thanks for your contribution.
As far as I see, the added tests all use the same union definition. Can you consolidate them into one test case containing multiple functions?

[chrchr-github]

If you wish, you can add yourself to the AUTHORS file. Please add [skip ci] in the commit message.

[DanTheMan2000]

If you wish, you can add yourself to the AUTHORS file. Please add [skip ci] in the commit message.

I don't think that's necessary. Thanks!